Calculator Inputs
Use the grid below. Large screens show three columns. Smaller screens show two. Mobile shows one.
Plotly Graph
The chart compares input values, gain ratios, and output voltages.
Example Data Table
| Case | V1 (V) | V2 (V) | R1 (Ω) | R2 (Ω) | R3 (Ω) | R4 (Ω) | Ideal Gain | Matched Output (V) |
|---|---|---|---|---|---|---|---|---|
| 1 | 0.10 | 0.90 | 1,000 | 10,000 | 1,000 | 10,000 | 10.00 | 8.00 |
| 2 | 0.20 | 1.20 | 2,200 | 22,000 | 2,200 | 22,000 | 10.00 | 10.00 |
| 3 | -0.10 | 0.40 | 4,700 | 47,000 | 4,700 | 47,000 | 10.00 | 5.00 |
| 4 | 0.50 | 1.10 | 10,000 | 100,000 | 10,000 | 100,000 | 10.00 | 6.00 |
Formula Used
1) Ideal Differential Gain
Ad = R2 / R1 when resistor ratios are matched.
2) Matched Difference Amplifier Output
Vout = (R2 / R1) × (V2 − V1) when R2/R1 = R4/R3.
3) General Output Equation
Vout = (1 + R2/R1) × [R4 / (R3 + R4)] × V2 − (R2/R1) × V1.
4) Supporting Input Terms
Differential input: Vd = V2 − V1
Common-mode input: Vcm = (V1 + V2) / 2
Matched resistor ratios improve subtraction accuracy. Ratio mismatch creates gain error and weakens common-mode rejection.
How to Use This Calculator
- Enter the inverting input voltage V1.
- Enter the non-inverting input voltage V2.
- Provide resistor values for R1, R2, R3, and R4.
- Click Calculate Gain.
- Review gain, output voltage, ratio mismatch, and input terms.
- Inspect the Plotly graph for a visual comparison.
- Use CSV or PDF export for lab or design records.
Frequently Asked Questions
1) What does this calculator measure?
It estimates differential op-amp gain and output voltage for a classic difference amplifier. It also shows resistor ratio balance, differential input, and common-mode input.
2) Why are R2/R1 and R4/R3 both important?
Both ratios should match for accurate subtraction. When they differ, the amplifier no longer behaves like an ideal differential stage, and output error increases.
3) What is differential input voltage?
Differential input voltage is the difference between the non-inverting and inverting inputs. It is written as Vd = V2 − V1.
4) What is common-mode voltage?
Common-mode voltage is the average of the two input voltages. It is written as Vcm = (V1 + V2) / 2.
5) Does this calculator include saturation effects?
No. It assumes ideal linear operation. Real devices can clip when output approaches supply rails or when input limits are exceeded.
6) Can I use kilo-ohms instead of ohms?
Yes, if you keep all resistor entries in the same unit. Ratios remain unchanged, so the gain result stays correct.
7) Why does the calculator show two output values?
One value uses the full general equation. The other uses the matched-ratio ideal equation. Comparing them helps reveal resistor mismatch impact.
8) When is this calculator most useful?
It is useful during circuit planning, lab work, instrumentation studies, sensor conditioning, and resistor network verification.